2,094 research outputs found
On the modelling of highly elastic flows of amorphous thermoplastics
Two approaches to the kinematic structuring of constitutive models for highly elastic flows of polymer melts have been examined systematically, assuming either: (1) additivity of elastic and viscous velocity gradients or (2) multiplicability of elastic and viscous deformation gradients. A series of constitutive models were compared, with differing kinematic structure but the same linear responses in elastic and viscous limits. They were solved numerically and their predictions compared, and they were also compared to those of the Giesekus model. Several variants, previously proposed as separate models, are shown to be equivalent and qualitatively in agreement with experiment, and therefore a sound basis for construction of models. But the assignment of viscous spin is critical: if it is assumed equal to the total spin with approach (1), or equal to zero with approach (2), then unphysical viscoelastic behaviour is predicted. © 2008 Elsevier Ltd. All rights reserved
Wall Adhesion and Constitutive Modelling of Strong Colloidal Gels
Wall adhesion effects during batch sedimentation of strongly flocculated
colloidal gels are commonly assumed to be negligible. In this study in-situ
measurements of colloidal gel rheology and solids volume fraction distribution
suggest the contrary, where significant wall adhesion effects are observed in a
110mm diameter settling column. We develop and validate a mathematical model
for the equilibrium stress state in the presence of wall adhesion under both
viscoplastic and viscoelastic constitutive models. These formulations highlight
fundamental issues regarding the constitutive modeling of colloidal gels,
specifically the relative utility and validity of viscoplastic and viscoelastic
rheological models under arbitrary tensorial loadings. The developed model is
validated against experimental data, which points toward a novel method to
estimate the shear and compressive yield strength of strongly flocculated
colloidal gels from a series of equilibrium solids volume fraction profiles
over various column widths.Comment: 37 pages, 12 figures, submitted to Journal of Rheolog
Pattern Formation and Elastocapillary Instabilities in Soft Gels
Hydrogels are complex materials that are typically utilized as bioinks in the emerging 3D bioprinting technology. Gels are distinguished by an elasticity that introduces complexity into the pinch-off process. The distinguishing feature of soft gels is that capillarity (surface tension) and elasticity are comparable which can induce an abundance of new phenomena in the elastocapillary regime. Unfortunately, elastocapillary instabilities are not fully understood because classical theories of fluid mechanics and solid mechanics can not capture the crossover between capillary-dominated and elasticity-dominated dynamics. Herein, elastocapillary surface phenomena on hydrogels are experimentally characterized and new theoretical models are proposed to interpret the discrepancies between classical theories and new experimental observations. Many first observations of dynamic elastocapillary phenomena are reported including the experimental observations of i) gel drop oscillations in ultrasonic levitation and ii) Faraday waves on mechanically-vibrated gels. The mechanism of pattern formation is investigated and the role of elasticity is revealed. By relating theory to experiment, a new diagnostic technique to measure the surface tension and rheology of soft gels is developed, which can directly support many emerging 3D bioprinting technologies
Frame Indifferent Formulation of Maxwell's Elastic Fluid and the Rational Continuum Mechanics of the Electromagnetic Field
We show that the linearized equations of the incompressible elastic medium
admit a `Maxwell form' in which the shear component of the stress vector plays
the role of the electric field, and the vorticity plays the role of the
magnetic field. Conversely, the set of dynamic Maxwell equations are strict
mathematical corollaries from the governing equations of the incompressible
elastic medium. This suggests that the nature of `electromagnetic field' may
actually be related to an elastic continuous medium. The analogy is complete if
the medium is assumed to behave as fluid in shear motions, while it may still
behave as elastic solid under compressional motions. Then the governing
equations of the elastic fluid are re-derived in the Eulerian frame by
replacing the partial time derivatives by the properly invariant (frame
indifferent) time rates. The `Maxwell from' of the frame indifferent
formulation gives the frame indifferent system that is to replace the Maxwell
system. This new system comprises terms already present in the classical
Maxwell equations, alongside terms that are the progenitors of the
Biot--Savart, Oersted--Ampere's, and Lorentz--force laws. Thus a frame
indifferent (truly covariant) formulation of electromagnetism is achieved from
a single postulate that the electromagnetic field is a kind of elastic (partly
liquid partly solid) continuum.Comment: accepte
Influence of supramolecular forces on the linear viscoelasticity of gluten
Stress relaxation behavior of hydrated gluten networks was investigated by means of rheometry combined with μ-computed tomography (μ-CT) imaging. Stress relaxation behavior was followed over a wide temperature range (0–70 °C). Modulation of intermolecular bonds was achieved with urea or ascorbic acid in an effort to elucidate the presiding intermolecular interactions over gluten network relaxation. Master curves of viscoelasticity were constructed, and relaxation spectra were computed revealing three relaxation regimes for all samples. Relaxation commences with a well-defined short-time regime where Rouse-like modes dominate, followed by a power law region displaying continuous relaxation concluding in a terminal zone. In the latter zone, poroelastic relaxation due to water migration in the nanoporous structure of the network also contributes to the stress relief in the material. Hydrogen bonding between adjacent protein chains was identified as the determinant force that influences the relaxation of the networks. Changes in intermolecular interactions also resulted in changes in microstructure of the material that was also linked to the relaxation behavior of the networks
Polymeric jets throw light on the origin and nature of the forest of solar spicules
Spicules are plasma jets, observed in the dynamic interface region between
the visible solar surface and the hot corona. At any given time, it is
estimated that about 3 million spicules are present on the Sun. We find an
intriguing parallel between the simulated spicular forest in a solar-like
atmosphere and the numerous jets of polymeric fluids when both are subjected to
harmonic forcing. In a radiative magnetohydrodynamic numerical simulation with
sub-surface convection, solar global surface oscillations are excited similarly
to those harmonic vibrations. The jets thus produced match remarkably well with
the forests of spicules detected in observations of the Sun. Taken together,
the numerical simulations of the Sun and the laboratory fluid dynamics
experiments provide insights into the mechanism underlying the ubiquity of
jets: the nonlinear focusing of quasi-periodic waves in anisotropic media of
magnetized plasma as well as polymeric fluids under gravity is sufficient to
generate a forest of spicules on the Sun.Comment: Published in Nature Physics. Video files are available at
https://rdcu.be/cZdl
Shear-induced transitions and instabilities in surfactant wormlike micelles
In this review, we report recent developments on the shear-induced
transitions and instabilities found in surfactant wormlike micelles. The survey
focuses on the non-linear shear rheology and covers a broad range of surfactant
concentrations, from the dilute to the liquid-crystalline states and including
the semi-dilute and concentrated regimes. Based on a systematic analysis of
many surfactant systems, the present approach aims to identify the essential
features of the transitions. It is suggested that these features define classes
of behaviors. The review describes three types of transitions and/or
instabilities : the shear-thickening found in the dilute regime, the
shear-banding which is linked in some systems to the isotropic-to-nematic
transition, and the flow-aligning and tumbling instabilities characteristic of
nematic structures. In these three classes of behaviors, the shear-induced
transitions are the result of a coupling between the internal structure of the
fluid and the flow, resulting in a new mesoscopic organization under shear.
This survey finally highlights the potential use of wormlike micelles as model
systems for complex fluids and for applications.Comment: 64 pages, 31 figures, 2 table
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